Composition and process for treating metal

ABSTRACT

A COMPOSITION AND PROCESS FOR THE TREATMENT OF FERROUS METAL SURFACES UTILIZING A NITRITE ACCELERATED ZINC PHOSPHATE TREATING SOLUTION, WHEREIN UNDESIRED CONCENTRATIONS OF FERRO-NITROSO COMPLEXES ARE PREVENTED BY INCORPORATING IN THE PHOSPHATING BATH A PEROXIDE CONTAINING SULFUR OXYGEN COMPOUND. EXEMPLARY OF PREFERRED COMPOUNDS WHICH MAY BE USED ARE NA2S2O8 AND H2SO5.

United States Patent US. Cl. 148-6.17 5 Claims ABSTRACT OF THE DISCLOSURE A composition and process for the treatment of ferrous metal surfaces utilizing a nitrite accelerated zinc phosphate treating solution, wherein undesired concentrations of ferro-nitroso complexes are prevented by incorporating in the phosphating bath a peroxide containing sulfur oxygen compound. Exemplary of preferred compounds which may be used are Na S O and H 50 This invention relates to an improved composition and process for the treatment of ferrous metal surfaces and more particularly it relates to improvements in the immersion phosphatizing of ferrous metals utilizing aqueous, acidic, nitrite-accelerated zinc phosphate solutions.

It has heretofore been known that an adherent zinc phosphate coating may be formed on ferrous metal surfaces by immersing the surfaces in a nitrite-containing aqueous zinc phosphate solution, at temperatures up to about 50 degrees C. In this process, the nitrite, which is generally added in the form of the alkali nitrite, serves to accelerate the rate at which the zinc phosphate coating layers are formed on the metal and, further, oxidizes the dissolved iron in the solution from ferrous to the ferrite state, thus effecting the precipitation of the iron from the solution in the form of the substantially insoluble ferric phosphate.

It has been found, however, that the amount of ferrous metal treated in these solutions, in a given period of time, is somewhat critical and that when the particular metal throughput rate is exceeded, in a given bath, the nitrite will no longer completely oxidize the dissolved ferrous iron in a reasonable period of time. When this occurs, there is an increasing brown discoloration of the treating bath, due to the formation of ferro-nitroso complexes. Additionally, the ferrous iron in the complex remains stable in the solution, even in the presence of added quantities of dissolved nitrites and may, in many instances, remain substantially unchanged for several hours. Moreover, as the metal throughput rate increases, a point is reached at which the solution is at no time free from the ferro-nitroso compounds. In this regard, it has been found that the concentration of these ferro-nitroso compounds increase with increased metal throughput rates, under conditions of decreasing nitrite concentration and bath temperature conditions.

This buildup, of the ferro-nitroso complexes in the treating solution has been found to be undesirable in that as the iron content in the bath increases, there is a corresponding increase in the probability that the phosphate coatings formed will contain ferrous phosphate. When this happens, there is an adverse effect on the anti-corrosion properties of the phosphate coating, as well as on the adhesive strength of the coating to the base metal. Additionally, because appreciable quantities of nitrous gases are bound in these complexes, when the coating baths are agitated, as for example when introducing or removing workpieces from the solution as well ice as during cleaning of the solutions, liberation of these nitrous gases occurs, creating a health hazard to the personnel working in the area.

Heretofore, it has been known to utilize a peroxide accelerated zinc phosphate solution for the coating of iron and steel and in German Pat. 881,893 it has been proposed to use peroxide with a nitrate preferably also adding additional quantities of nitrite to obtain thicker and softer coating layers. In this process, hydrogen peroxide has been used but when a metal throughput rate in excess of about0.16 square meter per liter of treating solution per hour is utilized. It has not been found to be effective in preventing the formation of ferro-nitroso complexes. Moreover, other peroxides such as the perborates and perphosphates have, likewise, not been found to be effective.

It is, therefore, an object of the present invention to provide an improved composition and process for forming a protective zinc phosphate coating on ferrous metal surfaces.

A further object of the present invention is to provide an improved process for forming zinc phosphate coatings on ferrous metal surfaces wherein the formation of ferro-nitroso complexes in the coating bath is substantially reduced.

A still further object of the present invention is to provide an improved nitrite accelerated zinc phosphate coating process which may be operated at substantially room temperature With high metal throughput rates without the undesirable formation of ferro-nitroso complexes.

These and other objects will become apparent to those skilled in the art from the description of the invention which follows.

Pursuant to the above objects, the present invention includes a composition for the treatment of ferrous metal surfaces which comprises an aqueous acidic zinc phosphate solution, which solution contains nitrite ions and peroxide, and wherein the peroxide content of the solution is provided by including in the solution at least one soluble sulfur-oxygen compound containing peroxogroups. When these solutions are used, even at room temperature, with relatively high metal throughput rates, it is found that the formation of stable ferro-nitroso complexes in these solutions are substantially reduced, if not eliminated completely.

More specifically, the phosphating solutions of the present invention are aqueous acidic zinc phosphate solutions which contain an accelerating amount of nitrite ions. Preferably, the nitrite content of these phosphatizing solutions are within the range of about 0.1 to 2 grams per liter, calculated as NaNO Amounts of nitrite outside of this preferred range may, of course, be used, depending upon the particular zinc phosphate solution and process operating conditions which are employed, so long as the amount of nitrite present is sufficient to effect the desired acceleration of the coating action and oxidation of the ferrous irons in the coating bath.

In addition to the nitrite, the phosphating solutions of the present invention also contain peroxide, in the form of one or more sulfur-oxygen compounds which contain peroxo-groups. These compounds will be present in the solution in amounts which will be effective in maintaining the solution substantially free from ferro-nitroso complexes, with amounts within the range of about 0.01 to 0.2 gram per liter, calculated as H 0 being preferred. Various sulfur-oxygen compounds containing a peroxo group may be used. Exemplary of suitable compounds of this type are peroxo-monosulfates, peroxo-disulfates, as well as the corresponding acids of these materials, such as the meta sulfuric acids, (Caros acid), as well as mixtures of these, and the like. Of these compounds, the preferred compounds have been found to be the alkali metal, and particularly the sodium peroxy disulfates (Na S O and the meta sulfuric acids (H 80 Accordingly, hereinafter particular reference will be made to these sulfur-oxygen compounds although, it is to be understood, that these are merely exemplary of similar materials which may be used.

In addition to zinc, the phosphating solutions of the present invention may also contain other layer-forming cations, such as manganese, calcium, and the like. Additionally, other oxidizing or accelerating agents, such as nitrate ions, copper ions, nickel ions, simple and/or complex fluorides, and the like, may also be present. Moreover, the solution may also contain one or more suitable wetting agents which will help promote the penetration of the phosphatizing solution into tightly packed workpieces, such as for example, wire coils. Additionally, the solutions may also contain lead ions, which materials have been found, in some instances, to be effective in promoting the generation of nitrite from nitrates in the solution.

It has been found that the advantageous effects of the sulfur-oxygen compounds in the present phosphating solutions are particularly realized when the solutions are used in immersion processes rather than spraying processes. This is not, however, to say that the present solutions cannot be used with spray application techniques but, rather, that in spraying applications, there is already sufficient agitation of the coating solution that the undesirable and detrimental ferro-nitroso complexes do not accumulate, even under extremely high metal throughput conditions. Additionally, it has been found that the advantages of the use of the solutions of the present invention are more greatly realized when using the solutions at relatively low temperatures. In this regard, it has been found that the preferred operating temperatures are at about room temperature or slightly above, with normal operating temperatures generally being below about 50 degrees C.

In this latter regard, it has been found that the tendency for the formation of the ferro-nitroso complex material is particularly strong at the lower temperatures. This fact is readily illustrated when steel wires were phosphated by immersion in a 45 point zinc phosphating solution containing copper, N0 lead, and about 0.7 gram per liter NaNO Under low metal throughput conditions of about 0.2 square meter per liter of solution over a 24 hour operating period, solution temperatures of about 30 degrees C. were found to be sufiicient to maintain the solution substantially free from the ferro-nitroso complexes, when the NaNO content was replenished to the level of 0.7 gram per liter. Where, however, the metal throughput rate was increased to about 0.6 square meter per liter in 24 hours, under the same operating conditions, solution temperatures had to be raised to 55 degrees C to maintain the solution free of the ferro-nitroso complexes. This method of operation, obviously, is an expensive way, in terms of heating costs, to maintain these solutions free of these ferro-nitroso complexes. Thus, in its most preferred embodiment, the phosphating solutions of the present invention are utilized in immersion processes, at temperatures which are not substantially in excess of about 50 degrees C.

It is to be appreciated that the sulfur-oxygen compounds may be used either in new phosphating baths or in solutions which are already in operation, and are effective in both in substantially minimizing the formation of the stable ferro-nitroso complexes. In the case of a new solution, the bath is made up in the conventional maner so as to contain the desired concentration of zinc, phosphate, as well as other components which may be required. Thereafter, before beginning the throughput of the metal to be treated, sodium nitrite is added to the bath, preferably in amounts within the range of about 0.3 to 0.7 gram per liter, along with the sulfur-oxygen compound, preferably in amounts within the range of about 0.03 to 0.06 gram per liter, calculated on the basis of H 0 As the bath is operated, zinc phosphate, the nitrite and the sulfuroxygen compounds are consumed. Accordingly, the zinc phosphate may be replenished in the conventional manner, using a replenishing concentrate containing the zinc and phosphate in amounts which will maintain the solution at a constant point level. In some instances, if desired, the concentrate containing the zinc and phosphate ions which is used to make up the initial solution may also be used as the replenishing concentrate. The nitrite ions which are consumed during operation may be replenished by adding to the solution amounts of sodium nitrite such that when a sample of the bath is titrated against a KMNO solution, the reducing effect will remain substantially constant. It is to be noted, however, that this relatively simple technique for determining nitrite may only be used with the baths of the present invention, which contain the sulfur-oxygen compound, and will not be satisfactory where hydrogen peroxide, per se, is added, as this will react with the KMNO With regard to the replenishment of the sulfur-oxygen compounds in the solution, this may be done very simply on the basis of a visual observation of the treating solution. When the solution remains brown-colored for a period of time in excess of about 10 to 20 minutes after the removal of the last ferrous metal workpiece which is being processed, the sulfur-oxygen materials should then be added to the solution in quantities as initially added to the bath before startup. Alternatively, however, a determination may be made as to the amounts of the sulfuroxygen compounds, as well as the sodium nitrite, which are consumed during the operation of the bath and a replenishing concentrate or mixture containing both of these components, in the weight ratio determined by their rate of consumption during use, may be added to the bath from time to time as is required to compensate for this usage. In this manner, the efficiency of the coating solutions may be continuously maintained. As has been previously noted, the process of the present invention may also be utilized with solutions which are already in operation and which already contain substantial quantities of the ferro-nitroso complexes. In this type of operation, a determination is first made as to whether the solution still contains suflicient quantities of nitrite to oxidize all of the ferrous iron to ferric iron. If not, then the required quantity of nitrite is added. Thereafter, preferably from about 0.1 to 0.2 gram per liter of the sulfuroxygen material, calculated as H 0 is added to the solution and this operation is repeated at about 15 minute intervals until the solution regains its light color. After this occurs, the bath may be operated in the manner described above, with periodic replenishment of all of the bath components, including the nitrite and the sulfur-oxygen compounds, as is required to maintain the efficient operation of the solution.

By operating the present compositions and processes in this manner, it is found that the formation of these undesirable ferro-nitroso complexes is substantially minimized if not completely eliminated. Moreover, it has been found that the phosphate coatings which are produced while operating in accordance with the process of the present invention are comparable to those produced in other processes of this type and are suitable for providing corrosion resistance, as well as lubrication in cold forming operation.

In order that those skilled in the art may better understand the present invention and the manner in which it may be practiced, the following specific examples are given. In these examples, unless otherwise indicated, parts and percent are by weight and temperatures are in degrees centigrade. It is to be appreciated, however, that these examples are merely exemplary of the present invention and are not to be taken as a limitation thereof.

5 EXAMPLE 1 An aqueous phosphatizing solution was prepared having the following components in the amounts indicated:

Components: Grams per liter Zinc 10.5

P 5.6 N0 18.1 NaNO 1.0 Na S O 1 1 Corresponding to 0.028 grams perllter H2011.

The total acidity of the solution was 30 points and the pH was adjusted with sodium hydroxide to 2.9. Steel sheets which had been pickled in sulfuric acid and rinsed with water were then immersed in the solution for minutes at 30 degrees C. The metal throughput in the solution was 0.04 square meter of steel surface per liter of solution. Following the removal of the sheets, the solution became completely light colored in less than one minute.

By way of comparison, a similar phosphating solution was prepared with the exception that the Na S O was re placed with 0.16 gram per liter of H 0 Steel sheets were than treated in this solution in the same manner as previously set forth. Following the removal of the sheets from the solution, the solution was found to be dark brown in color and this dark brown color remained constant for about 30 mintues.

EXAMPLE 2 An aqueous phosphatizing solution was prepared con taining the following components in the amounts indicated:

Components: Grams per liter Zinc 14.7

P 0 9.7 No 24.7 Pb 0.025

NaNO 0.7

This solution had a total acidity of 40 points. The solution was made up in three portions. To the first was added 0.07 gram per liter H 0 to the second 0.2 gram per liter NaBO .H O (corresponding to 0.07 gram per liter H 0 and to the third 0.5 gram per liter Na S O- (corresponding to 0.07 gram per liter H 0 Drawing lubricant was removed from steel wire segments by treatment in sulfuric acid, and after rinsing in water, the segments were immersed in the various solutions for 10 minutes at 30 degrees C. The metal throughput through each of the solutions was 0.04 square meter of steel surface per liter of solution. Following the removal of the wire segments from the solutions, it was found that the solutions to which the H 0 and the NaBO .H O were added were brown colored and this brown color remained constant for more than 30 minutes. In contrast, the solution to which the Na S O had been added became clear in less than one minute.

EXAMPLE 3 To show the effect of the addition of various amounts of hydrogen peroxide to these phosphating solutions, a phosphating solution was formulated as described in Example 2.*To different portions of this solution was added hydrogen peroxide (H 0 in amounts of 0.04, 0.07, 0.10, 0.16, 0.5, 1.0 and 3.0 grams per liter. Steel wire segments which had been pretreated in the manner described in Example 2 were then phosphated in accordance with the process set forth in this example. Following the removal of the wires from the various solutions it was found that the solutions which contained hydrogen peroxide additions in quantities up to and including 0.16 gram per liter were all brown colored and this brown coloration remained for over 30 minutes. In those solutions which had hydrogen peroxide additions of 0.5 gram per liter and higher were not brown colored but there was no phosphate coating formed on the wire segments treated in these solutions.

EXAMPLE 4 By way of comparison, the procedure of Example 3 was repeated with the exception that instead of the hydrogen peroxide, Na S O was added to the various portions of the treating solution in amounts of 0.1, 0.03, 0.5, 0.7 and 1.3 grams per liter. These additions corresponded to 0.014, 0.043, 0.071, 0.10, and 0.16 gram per liter H 0 The steel wire segments were then processed in the solutions in the same manner as in Example 3 and following the removal of the wires from the solution it was found that the time for reestablishment of the light color in the solution containing 0.1 gram per liter Na S O was 20 minutes while in all the other solutions became clear in less than one minute. Additionally, the phosphate coatings formed on the wires treated in each of the solutions were all found to provide uniform coverage with no visible difference in the coatings formed, the coating weights all being from about 10 to 11 grams per square meter.

The above procedure was again repeated with the exception that instead of the Na S O corresponding quantities of meta-sulfuric acid (H 50 Were used. The results obtained were identical to those obtained with the Na S O additions.

EXAMPLE 5 An aqueous phosphatizing solution was prepared containing the following components in the amounts indicated:

Component: Grams per liter Zinc 7.2

P205 NaNO 0.7

This solution had a total acidity of 40 points. One portion of this solution was designated as solution A, while a second portion, designated as solution B, was modified by the addition thereto of 0.2 gram per liter Na S O Steel wire segments were treated in sulfuric acid to remove any lubricant materials therefrom, rinsed with water and then phosphated by immersion for 10 minutes at 30 degrees C. in the two solutions, using a throughput frequency of 0.2 square meter per liter of solution for an hour. The solutions were replenished to the indicated point level using a concentrate containing 6.52% by weight zinc and 17.30% by weight P 0 and by using NaHO to maintain the consistency of the reducing effect of the solution against KMNO Additionally, after each throughput of 0.04 square meter of steel surface per liter of solution, solution B was replenished with 0.2 gram per liter Na S O The phosphate coatings obtained from both solutions A and B were strongly adherent and quite satisfactory. In solution A, however, after a throughput of only 0.12 square meter of steel surface per liter of solution, a brown coloration developed in this solution, which coloration remained and was not disapated during the remainder of the run, despite replenishing of the solution with sodium nitrite. In contrast, however, four square meters of steel surface per liter of solution were passed through solution B without any discoloration developing in the solution which lasted longer than about 1 minute.

While there have been described various embodiments of the invention, the compositions and methods described are not intended to be understood as limiting the scope of the invention as it is realized that changes therewithin are possible and it is intended that each element recited in any of the following claims is to be understood as referring to all equivalent elements for accomplishing substantially the same results in substantially the same or equivalent manner, it being intended to cover the inven- 7 tion broadly in whatever form its principle may be utilized.

What is claimed is:

1. A composition suitable for treating ferrous metal surfaces which comprises an aqueous acidic zinc phosphate solution, which solution contains nitrite ions present in an amount within the range of 0.1 to 2 grams per liter calculated as NaNO and peroxide wherein the peroxide is incorporated in the solution by the addition of a peroxomonosulfur compound being present in an amount within a range of about 0.01 to 0.2 gram per liter, calculated as H 0 and is sufficient to minimize the formation of ferro-nitroso complexes during the use of the bath in the treatment of ferrous surfaces.

2. The composition of claim 1 wherein the peroxomonosulfur compound is selected from the group consisting of peroxo-monosulfates and peroxo-monosulfuric acid.

3. A process for phosphating ferrous metal surfaces which comprises immersing the ferrous metal to be treated v in the phosphatizing solution as claimed in claim 1 and maintaining the ferrous metal in the solution for a period sufiicient to form a phosphate coating thereon.

4. The process of claim 3 wherein the peroxo-monosulfur compound is selected from the group consisting of peroxo-monosulfates and peroxo-monosulfuric acid.

5. The process as claimed in claim 4 wherein the phosphatizing solution is maintained at a temperature which is not substantially in excess of about 50 degrees C.

References Cited UNITED STATES PATENTS OTHER REFERENCES Shaw et al., Ger. Offen 1941489, Feb. 19, 1970, CA 72 P103264C.

20 RALPH S. KENDALL, Primary Examiner US. Cl. X.R. l486.l5 Z 

